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sometimes in a regular vertical layer, and sometimes in irregular and detached masses. I shall therefore occasionally, without wishing to prejudge the question of the formation of veins, speak of the fissures in which they are deposited.

α.

The direction of the intersection of a vein with a horizontal plane usually approximates to rectilinearity. It is not meant that every short portion of this intersection forms a straight line, but, when considered with reference to its whole extent, these variations are not for the most part considerable.

B. In every mining district the largest and most important veins are divided into two distinct groups, in each of which a very decided approximation to parallelism is observable, and of which the directions are nearly perpendicular to each other.

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When the veins occur in stratified masses, the direction of one of these systems usually coincides with that of the general dip of the strata, the other being consequently perpendicular to that direction*.

S. A large proportion of the most productive mineral veins are found in the former of these systems. The latter (frequently termed by the miner cross courses) carry ore very irregularly.

€.

It seems doubtful whether any actual limits of a fissure containing a mineral vein were ever arrived at by the miner, though the division of a large fissure into several small ones not unfrequently seems to indicate a near approach to such a limit in the direction of its length. I know of no case, however, in which such indications have been observed of an approach to both extremities of a large vein. It is probable that their linear extent is frequently much greater than has yet been, or in many cases ever can be, observed. In numberless instances they

* I first observed this relation between the general direction of the mineral veins and that of the dip of the strata in the mining district of Derbyshire. I find on enquiry that the same relation holds in the Alston-moor district, and in Flintshire. In Cornwall also, when the lodes are in stratified rock, I apprehend this is generally the case, assuming the killas formation in the immediate vicinity of the granite to be stratified.

have been traced for four or five miles in the mining districts of this country, and in some cases to the distance of eight or ten miles.

3. Their depth appears to be uniformly greater than that to which man has been able to penetrate.

n. The width of the fissures in that system of the two above mentioned which contains the most productive and the most continuous mineral veins, varies in general from a few inches to about 12 feet. In the same vein the width will frequently vary, and sometimes suddenly along the same vertical line. In passing through a horizontal bed of clay the fissure will be sometimes almost entirely closed; and the toadstone of Derbyshire produces the same effect, frequently closing the fissure so effectually that it can only be traced through it by means of small ramifying veins of calcareous spar. The average width however does not appear at all to diminish as we descend*. The strata through which the fissure penetrates generally form well defined though uneven walls bounding it on either side, and perfectly firm and solid, except where the strata themselves cease to be so.

0. The width of the cross courses is frequently greater than that above stated, and generally much more irregular.

L.

The fact of the strata in one wall of a fissure being higher than the same strata in the opposite one, has been recognized by all miners in some parts of almost every vein of consequence that has been explored, when existing in a distinctly stratified mass. This difference in general does not exceed a few feet, though it has not unfrequently been found to be many fathoms, in which case the vein of course coincides with a fault. This is sometimes termed by miners the throw of the vein.

* In the mining district about Alston-moor there appears to be a few exceptions to this rule, as well as to the assertion of the preceding paragraph (C), in what are termed gash veins. These are comparatively wide at the top, and become gradually narrower as they descend, till they appear to terminate. (See Forster's account of this district, p. 186.) They are probably rents the formation of which began at the surface, but are hardly worthy of notice as exceptions to our general rules.

+ A throw is in fact a small fault.

K.

The inclination of the plane of the fissure to a vertical plane, which is frequently termed by the miners of the more northern districts the hade of the vein, and by the Cornish miners its underlie, is very uncertain, amounting not unfrequently to as much perhaps as 20°, generally, however, to considerably less, though in particular cases to considerably more. It will sometimes vary at different depths along the same vertical line, so that in some instances, when the hade is small, it will be in one direction in the upper, and in the contrary direction in the lower part of the vein. Upon the whole, however, the hade is not very great, and tolerably regular in each vein*.

μ. Masses of the adjoining rock, more or less perfectly detached from it, are frequently found imbedded in the matter which occupies the fissure t.

ν.

Apparent or real displacements in the position of a vein are frequently observed at its intersection with another vein, or with some particular stratified bed, which is generally found to be a bed of moist slimy clay. These intersections are of various kinds.

0.

First, that of a vertical or nearly vertical vein, with a clay bed horizontal, or nearly so. The displacements in this case are shewn in the figures annexed, which represent vertical sections perpendicular to the plane of the vein.

It is manifest that here either the part of the vein above the stratum. cd has been moved, or that below it, or both, if the two portions were ever in the same plane.

The underlie of the Cornish lodes is frequently greater, I conceive, than in our other mining districts. It may possibly also be more irregular.

+ These insulated masses are frequently termed by miners, Riders.

T. Secondly, we may have the intersection of two vertical veins, the planes of which are inclined to each other at any given angle. In such case it frequently happens, that while the continuity of one vein is preserved that of the other is broken, apparently by a relative displacement of the portions on opposite sides of the unbroken vein. This kind of displacement is exhibited in the annexed figures, which represent horizontal sections.

p. Thirdly, we may have the intersection of veins the planes of which are inclined, but at different angles, to a vertical plane. If such veins be near enough to each other, their intersection will take place sufficiently near the surface to be within the limits of observation, and if they meet the horizontal surface in parallel lines their line of intersection will be horizontal. If the subjoined figures represent vertical

sections perpendicular to this line, the displacements observed will be such as they exhibit.

These phenomena of faults, and mineral veins, are those which appear to approximate the most distinctly to well defined laws, and therefore afford the best means of testing the truth of any theory of elevation. The following phenomena also bear equally on the investigations con

tained in this paper, though their characters are in general much less distinct than those of the phenomena already cited.

III. Anticlinal and Synclinal Lines.

When two or more anticlinal lines, with the corresponding synclinal ones, are found in the same geological district*, their general directions frequently approximate to parallelism with each othert.

IV. Longitudinal Valleys.

a. Along the flanks of elevated ranges, longitudinal valleys are not unfrequently found running nearly parallel to the general axis of elevation +.

B. The partial elevations along the sides of an elevated range have usually these escarpments presented towards the central ridge ||.

I mean by a geological district, any tract of country throughout which the phenomena may be regarded as following the same laws without discontinuity.

+ If we take two planes coinciding at any proposed point of an anticlinal line, with the portions of the surface of a stratified bed on opposite sides of that line, these planes of stratification will intersect in a straight line not necessarily horizontal; and the direction of the anticlinal line at the proposed point will be determined by the azimuth of a vertical plane drawn through this intersection, or the direction of the intersection of this vertical plane with the horizon. Again, if through the proposed point we draw vertical planes respectively perpendicular to the two planes of stratification above mentioned, their respective intersections with them will be the lines of greatest inclination of the strata, and consequently the azimuths of these vertical planes will determine the directions of the dip. The angles between these two latter vertical planes, and the one before mentioned as determining the direction of the anticlinal line, will not generally be equal; they will become so only when the inclination of the planes of stratification on either side of the line is the same; i. e. the directions of the dip on opposite sides of an anticlinal line at any proposed point of it will not generally make equal angles with that of the line itself, unless the dip on opposite sides be the same. There is however an exception to this rule, when the direction of the dip on each side of the anticlinal line is perpendicular to it. This will occur when the two planes of stratification first mentioned intersect in a horizontal line. Saussure, Voyages dans les Alpes, Vol. I. Chap. x.

Traité de Geognosie, by D'Aubuisson, Vol. I. §. 24. p. 82.; and Saussure, Voyage dans les Alpes, Vol. III. Chap. x. This rule is probably very general.

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